A real time executive system for manned spaceflight

The Real Time Executive Control System discussed in this paper was the foundation for the applications programs developed in support of NASA's Gemini and early Apollo missions. Services provided by the Executive included dynamic storage management and allocation, two-level priority multiprogramming, real time data control and routing, real time error recovery, dynamic statistical monitoring, debugging facilities, and the program linkages and services that facilitated modular and independent applications system design. While a selection of these services may be available in other systems, the Executive design differs from other real time systems by these characteristics: • Modularity---The Executive design permitted the addition of new services and facilities based on equipment changes or applications requirements with no impact on the previously provided services and facilities. • Simplicity --- Only a minimal instruction in Executive services was necessary before applications programmers could construct programs that operated in a complicated real time environment. • Versatility --- Executive could be used in the simplest simulated real time environment for the debugging of one applications program or the support of the most demanding real time missions. • Generality --- Executive was non-applications oriented; i.e., it operated equally well in a real time Gemini mission, an astronaut training session, or in a non-real time environment using simulated input from tapes. • Invulnerable---The Executive was virtually unstoppable in real time; a feature vital for manned spaceflight.